Converting Volume Units: Litres, Gallons and Cubic Measures
Volume has the single worst trap in everyday measurement: there are two gallons in common use and they differ by nearly a fifth. The Volume Converter handles both, along with litres and cubic units.
The two gallons
A US gallon is 3.785 litres. An imperial gallon is 4.546 litres. The imperial one is about twenty per cent larger, and both are called a gallon.
So a figure quoted in gallons is meaningless without knowing which. A car doing 30 miles per gallon in Britain is doing 25 in American terms, for exactly the same fuel consumption - a difference that makes British cars look better than they are in any careless comparison.
The imperial gallon survives mainly in Canada and the Caribbean for fuel, and in British recipes and pub measures. The United States uses its own. Nowhere else uses either.
Fluid ounces follow the gallons and are worse: a US fluid ounce is 29.57 ml, an imperial one 28.41 ml - so the imperial ounce is smaller while the imperial gallon is larger, because there are 160 imperial fluid ounces to the gallon and only 128 US ones.
Cubic units get cubed
A cubic metre is 1,000 litres, and it is 35.3 cubic feet rather than 3.28. The length factor is cubed for volume, the same trap as squaring for area.
A litre is exactly a cubic decimetre - a cube ten centimetres on a side - which is the neatest thing about the metric system. A millilitre is a cubic centimetre, so 1 ml of water is 1 cubic centimetre and weighs 1 gram. That triple identity is why scientific work is done in metric and not out of national preference.
Volume and weight are not interchangeable
A litre of water weighs a kilogram. A litre of oil weighs about 0.92 kg, a litre of honey about 1.42, a litre of flour about 0.6. So converting a volume to a weight needs the density of the specific substance, which no volume converter can know.
This is the thing that ruins recipes across the Atlantic, because American baking measures by volume in cups and European baking by weight. A cup of flour can vary by twenty per cent depending on whether it was scooped or spooned, which is why the Cooking Measurement Converter is a separate tool and why weighing flour is better than measuring it.
Common questions
How many litres in a gallon?
3.785 US, 4.546 imperial. Always ask which.
Is a pint a pint?
No. A US pint is 473 ml, an imperial pint 568 ml. The British pint is noticeably larger, which bar staff on both sides will confirm.
How much is a cubic foot?
About 28.3 litres, or 0.028 cubic metres.
What about a barrel?
Depends entirely on what is in it. An oil barrel is 159 litres; beer and other barrels are different again.
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